Asymmetric synthesis of a tertiary arsine by nucleophilic addition to a chiral phosphine-stabilized arsenium salt

A new method for the asymmetric synthesis of As-chiral tertiary arsines is described. The addition at -95 degrees C of n-butyllithium to a dichloromethane solution of a phosphine-stabilized arsenium salt of the type (+/-)-[R3P -> AsMePh]PF6, where R3P is an enantiomerically pure, atropisomeric phosphepine derived from lithiated (aR)-2,2'-dimethyl-1,1'-binaphthalene, furnishes (S-As)-(+)-(n-butyl)methylphenylarsine in 85% enantioselectivity (70% enantiomeric excess) with displacement of the (aR(P))-phosphepine. The enantioselectivity of the synthesis is lower than the diastereoselectivity of coordination of the (aR(P))phosphepine to the prochiral, six-electron methylphenylarsenium ion with which it is in equilibrium in solution by P-As bond dissociation, 94% diastereomeric excess, as determined by NMR spectroscopy at -95 degrees C. The excess of the S enantiomer of the arsine, however, is consistent with the S(N)2 mechanism proposed for the reaction and the solution and solid-state structures of the predominant diastereomer of the phosphepine-arsenium complex. A feature of the design of the phosphepine auxiliary is the attachment of a 2-(methoxymethyl)phenyl substituent at phosphorus, the oxygen of which interacts with the arsenic of the arsenium ion, in solution and in the solid state, and facilitates stereodifferentiation by the chiral phosphepine of the enantiotopic faces of the prochiral, six-electron methylphenylarsenium ion; the significance of this anchimeric interaction is borne out by DFT calculations on a closely related model complex.